Biodegradable resin, biodegradable resin composition, biodegradable molded object, and process for producing biodegradable resin

ABSTRACT

A biodegradable resin having sufficient heat resistance, molding property and recycling property can be obtained, without losing biodegradability, by introducing a covalently bonded and thermo-reversible cross-linked structure into a biodegradable resin. Heat resistance, molding property, recycling property and biodegradability can be further improved, if necessary, by setting the cleaving temperature of a cross-linked structure in a given range, selecting the kind of a cross-linked structure, and making a three-dimensional cross-linked structure.

TECHNOLOGICAL FIELD

The present invention relates to a resin and a resin composition,rendering material recycle easy by using a thermo-reversiblecross-linking method, having excellent heat resistance and moldingprocessability, and having biodegradability, and a method of producingthem.

BACKGROUND TECHNOLOGY

Plastics are used in large amount in wide industrial fields because ofexcellent properties such as easy shaping, lightweight, cheapness,durability and the like. However, due to durability, even if usedplastics are discarded into the natural world, these are not decomposedto cause an environmental problem, in some cases. Since used plasticsare not permitted to be discarded into the natural world, they should besubjected to a burning treatment and the like after use, however, due tolarge heat generation in burning, there is a possibility of injuring acombustion furnace in burning, and dioxin is generated by burning, insome cases. Based on such facts, there is desired a biodegradableplastic which can be recycled and which is decomposed by a microorganismand the like when discarded into the natural world after use.Particularly, from the standpoint of reduction of production energy anddecrease in discharge amount of carbon dioxide, a material-recyclablebiodegradable plastic is more desired than a thermal-recyclablebiodegradable plastic.

However, conventional biodegradable plastics are insufficient inproperties such as heat resistance and the like as compared with generalplastics, in some cases. Therefore, for the purpose of improving theproperties of a biodegradable plastic, such as heat resistance and thelike, Japanese Patent Application Laid-Open (JP-A) No. 6-192375 suggestsa technology in which polycaprolactone is cross-linked with anisocyanate, and the heat resistance of a biodegradable plastic isimproved by introducing a cross-linked structure of a covalent bond.

In the above-mentioned conventional technology, the heat resistance andthe like of a biodegradable plastic are improved by a cross-linkedstructure, however, there are a possibility of decrease in flowabilityin heat melting, a possibility of insufficient moldability, and apossibility of decrease in biodegradability. Particularly, in the caseof a highly cross-linked biodegradable plastic, when this is oncemolded, it behaves as if a thermosetting resin, and even if this is tobe recovered and recycled, sufficient heat melting is not attained insecond and later moldings, leading to difficult recycling, in somecases.

For the purpose of improving a recycling property, there is a suggestionon introduction of a thermo-reversible cross-linked structure withcovalent bond into a plastic. First, as examples of a thermo-reversiblereaction based on covalent bond, Engle et al., J. Macromol. Sci. Re.Macromol. Chem. Phys., vol. 33, no. C3, pp. 239 to 257, 1993 describes aDiels-Alder reaction, nitroso dimerization reaction, esterificationreaction, ionene-forming reaction, urethane-forming reaction andazlactone-phenol addition reaction.

Nakane Yoshinori and Ishidoya Masahiro, et al., Shikizai (ColoringMaterial), vol. 67, No. 12, pp. 766 to 774, 1994; Nakane Yoshinori andIshidoya Masahiro, et al., Shikizai (Coloring Material), vol. 69, No.11, pp. 735 to 742, 1996; JP-A No. 11-35675, describe athermo-reversible cross-linked structure utilizing a vinyl ether group.

Further, there are examples as described below for obtaining a recyclingproperty utilizing a thermo-reversible cross-linked structure withcovalent bond.

JP-A No. 7-247364 describes a method for separating and recovering anoligomer and chemically-recycling this, utilizing a reversiblycross-linkable oligomer, and describes, as a method of cleaving across-linked portion, a means for irradiation with ultraviolet ray and ameans for cleaving by heat utilizing a Diels-Alder reaction. However,for conducting a cleaving reaction uniformly utilizing light, it isdifficult for a molded article itself to secure transparency againstlight and it is necessary to dilute and dissolve the molded article inan organic solvent before the reaction, and this procedure has extremelypoor efficiency as compared with a usual material recycle of resin byheat melting. According to Example 3 of this publication, a cleavingreaction by heat occurs at 90° C. This cleaving temperature is equal toor lower than the glass transition temperature (90 to 105° C.) of aresin (polyacrylate) as a mother material, rather deteriorating heatresistance. When intending improvement in sufficient heat resistance at100° C. or more, it is necessary that a cross-linked portion cleavingreaction occurs at temperatures at least over 120° C. Therefore, it isnecessary to select a reversible cross-linking portion having suitablecleaving reaction temperature and apply this to a resin.

Japanese Patent Application National Publication (Laid-Open) No.10-508655 attains a recycling property by introducing2,5-dialkyl-substituted furan into a resin. Introduction of furan isperformed by a dehydration reaction of a copolymer of carbon monoxidewith an olefin, with a strong acid. However, in the case a biodegradableresin, it is polymerized by an easily-hydrolyzable functional group suchas an ester bond and the like. Introduction of a furan ring by suchmeans is very difficult since decomposition of a resin is caused bythis. The cleaving temperature of a cross-linked portion and the thermalstability of a diene depend significantly on the polarity andconcentration of a reaction field. In the case of biodegradable resin,it is not necessary to limit the Diels-Alder reaction to those using2,5-dialkyl-substituted furan.

Further, examples utilizing a reversible reaction by an esterificationreaction of an acid anhydride for improvement in heat resistance andimprovement in a recycling property are described in JP-A No. 11-106578,and the like, and a means is shown in which a carboxylic anhydride isintroduced into a vinyl polymerization compound and cross-linked with alinker having a hydroxyl group. However, a lot of biodegradable resinshave in the main chain a hydrolysable bond in which a carboxylic acidacts as a catalyst, such as an ester bond and the like. When theesterification reaction of an acid anhydride is introduced into abiodegradable resin, a free carboxylic acid is generated when across-linked portion has been formed, and the hydrolysis speed of abiodegradable resin as a mother material becomes remarkably high inpreservation of a resin before molding and in use of a molded article,consequently, the moisture resistance and durability of the resin lowermore than the necessity and the resin cannot be practically used.

On the other hand, in the case of a carboxy-alkenyloxy type, when acompound having a bond cleaving temperature of 120° C. or more is used,a free carboxylic acid is not generated easily at practical temperaturesof 100° C. or less. When a resin is previously dried sufficiently inmolding, hydrolysis does not occur and its durability is notdeteriorated. Further, a nitroso dimer type, urethane type andazlactone-hydroxyaryl type are also applicable.

There is also an example of introduction of a thermo-reversiblecross-linked structure by an electrostatic bond into a biodegradableresin. First, as examples of electrostatic bond, there are JP-A No.2000-281805 and, Yano Shinichi, Ionomer no Bussei to Kougyouteki Ouyou(Physical Property and Industrial Application of Ionomer); M. R. Tant etal., Ionomers (ISBN: 0-7514-0392-X).

As an example utilizing a thermo-reversible cross-linked structure by anelectrostatic bond in a biodegradable resin, JP-A No. 2000-281805discloses an ion-crosslinked film obtained by cross-linking a carboxylgroup of polysaccharides such as carboxymethylcellulose having acarboxyl group, carboxyl group-containing starch and the like with apoly-valent metal ion, for the purpose of improving strength. However,in general, an electrostatic bond is inferior in bonding strength to acovalent bond, consequently, heat resistance cannot be desired to besufficiently improved, though the viscosity and elastic modulus of aresin are improved remarkably.

As described above, there are a lot of trials for realizing therecycling property of a resin material by introducing athermo-reversible cross-linked structure with covalent bond in the resinmaterial, however, there is few example applying this in a biodegradableresin material. It is technologically difficult to introduce athermo-reversible cross-linked structure with covalent bond into abiodegradable resin material, and is has been difficult to realizepractical properties with a recyclable biodegradable resin material.

SUMMARY OF THE INVENTION

In view of the above-mentioned conditions, the present invention has anobject of providing a recyclable biodegradable resin material havingpractical properties by introducing a thermo-reversible cross-linkedstructure with covalent bond into a biodegradable resin material.

More specifically, the present invention has an object of providing aresin and a resin composition having sufficient heat resistance, moldingproperty, recycling property and biodegradability.

According to the present invention for obtaining the above-mentionedobject, a biodegradable resin is provided having a functional groupforming a thermo-reversible cross-linked structure which is covalentlybonded by cooling and cleaved by heating.

Further provided is a biodegradable resin composition comprising a firstbiodegradable resin having a first functional group forming athermo-reversible cross-linked structure which is covalently bonded bycooling and cleaved by heating, and a second biodegradable resin havinga second functional group forming a thermo-reversible cross-linkedstructure which is covalently bonded with the first functional group bycooling and cleaved by heating.

Furthermore provided is a biodegradable resin composition comprising afirst biodegradable resin having a first functional group forming athermo-reversible cross-linked structure which is covalently bonded bycooling and cleaved by heating, and a linker having a second functionalgroup forming a thermo-reversible cross-linked structure which iscovalently bonded with the first functional group by cooling and cleavedby heating.

Further provided is a method of producing a biodegradable resincomprising a step of reacting a cross-linking agent having a structureof the covalent bond of a first functional group and a second functionalgroup, which is covalently bonded by cooling and cleaved by heating, anda third functional group, with a biodegradable resin material having asite reacting with the third functional group.

Even further provided is a method of producing a biodegradable resincomprising a step of cross-linking a first biodegradable resin having afirst functional group forming a thermo-reversible cross-linkedstructure which is covalently bonded by cooling and cleaved by heating,with a linker having two or more second functional groups forming athermo-reversible cross-linked structure which is covalently bonded withthe first functional group by cooling and cleaved by heating.

The cross-linked structure of a biodegradable resin having athermo-reversible cross-linked structure is cleaved in melt molding.Accordingly, even if a portion composed of cross-linked structures in anumber necessary for rendering properties such as heat resistance andthe like sufficient is introduced, viscosity is suitable in melting andexcellent molding processability can be realized. Even if this is moldedonce, the molded body does not behave like a thermo-setting resin, andwhen this is recovered and recycled, it can be thermally-meltedsufficiently even in second or later moldings, realizing an excellentrecycling property. Further, since it is solidified in cooling to againform a cross-linked structure, the molded body has sufficient heatresistance.

Particularly, since a covalently bondable thermo-reversible cross-linkedstructure has suitable bonding force as compared with athermo-reversible cross-linked structure by electrostatic bond, if thisis introduced into a biodegradable material, heat resistance andstrength and the like which are inferior in conventional biodegradableresin material can be improved due to the cross-linked structure underuse environments while the cross-linked structure is cleaved to securehigh flowability in molding at high temperatures.

In the case of thermo-reversible cross-linked structure, after cleavageof a cross-linked structure at high temperatures, the cross-linkedstructure is again formed by a cooling operation conducted subsequently.Therefore, the cleaving and re-formation of a cross-linked structure canbe repeated for any times by temperature changing. By introducing such across-linked structure into a biodegradable resin material, an excellentresin and resin composition can be obtained. Namely, at temperatures foruse of a molded body such as normal temperature and the like, it ispossible to form a super-cross-linked structure to improve heatresistance and strength, and at temperatures equal to or higher than themelting point such as molding temperature and the like, a cross-linkedstructure is lost and the molecular weight of the resin lowers,consequently, flowability increases, and a molding property and materialrecycling property can be improved.

The molded body, when solidified, contains mainly a resin cross-linkedby covalent bond, and when melted, the cross-linked portions is cleaved,therefore, a composition containing two or more resins is obtained, or acomposition containing a resin and a linker is obtained, in some cases.Therefore, when it is not necessary to specially distinguish a resin anda resin composition, there are also referred to as resin substance.

Further high performance and wide physical properties can be realized byintroducing an electrostatic bondable thermo-reversible cross-linkedstructure into a biodegradable resin, in addition to covalently bondablethermo-reversible cross-linked structure. Specific examples thereofinclude a method in which a functional group forming a covalentlybondable cross-linked structure and a functional group forming across-linked structure with electrostatically bonding are introducedinto the same biodegradable resin material; a method in which abiodegradable resin material containing an introduced functional groupforming a covalently bondable cross-linked structure and biodegradableresin material containing an introduced functional group forming across-linked structure with electrostatically bonding are mixed; amethod in which a functional group forming a cross-linked structurehaving both a covalent binding and an electrostatic bonding isintroduced; and the like.

A cross-linked structure with electrostatically bonding is biodegradedquickly when buried in soil and the like in the presence of water.

As a result, a covalently bondable thermo-reversible cross-linkedstructure can be introduced into a biodegradable resin material, andpractical properties can be realized in a recyclable biodegradable resinmaterial.

Consequently, a resin and a resin composition having sufficient heatresistance, molding property, recycling property and biodegradabilitycan be realized.

Additionally, heat resistance, molding property, recycling property andbiodegradability can be further improved by carefully selecting the kindof a biodegradable resin material, setting the cleaving temperature of across-linked structure in given range, carefully selecting the kind of across-linked structure, and making a three-dimensional cross-linkedstructure.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention will be illustrated in detail below.

(Biodegradable Resin Material)

The biodegradable resin material as a raw material of a biodegradableresin is selected while sufficiently considering the nature of afunctional group introduced to form a thermo-reversible cross-linkedstructure.

As such a biodegradable resin material, there are biodegradablemonomers, oligomers and polymers which are mainly artificiallysynthesized and available; derivatives of biodegradable monomers,modified bodies of biodegradable oligomers and modified bodies ofbiodegradable polymers which are mainly artificially synthesized andavailable; biodegradable monomers, oligomers and polymers which aremainly naturally synthesized and available; derivatives of biodegradablemonomers, modified bodies of biodegradable oligomers and modified bodiesof biodegradable polymers which are mainly naturally synthesized andavailable; and the like.

Examples of the artificially synthesized biodegradable oligomer andpolymer include poly-α-hydroxy acids such as polylactic acid(manufactured by Shimadzu Corp., trade name: Lacty, and the like),polyglycolic acid and the like, polyesters such aspoly-ω-hydroxyalkanoate such as poly-ε-caprolactone and the like(manufactured by Daicel Chemical Industries, Ltd., trade name: Plaxel,and the like), polyalkylene alkanoates as a polymer of butylenessuccinate and/or ethylene succinate (manufactured by Showa KobunshiK.K., trade name: Bionole, and the like), polybutylene succinate and thelike, polyamino acids such as poly-γ-glutamate (manufactured byAjinomoto Co., Inc., trade name: Polyglutamic acid, and the like) andthe like, polyols such as polyvinyl alcohol, polyethylene glycol and thelike.

Modified bodies of these artificially synthesized biodegradableoligomers and polymers can also be suitably used.

The naturally synthesized biodegradable oligomers and polymers includepolysaccharides such as starch, amylose, cellulose, cellulose ester,chitin, chitosan, Guerin gum, carboxyl group-containing cellulose,carboxyl group-containing starch, pectinic acid, alginic acid and thelike; poly-β-hydroxy alkanoates as a polymer of hydroxy butyrate and/orhydroxy valerate synthesized with a microorganism (manufactured byZeneca, trade name: Biopole, and the like) and the like, and of them,preferable are starch, amylose, cellulose, cellulose ester, chitin,chitosan, poly-β-hydroxy alkanoates as a polymer of hydroxy butyrateand/or hydroxy valerate synthesized with a microorganism, and the like.

Modified bodies of naturally synthesized biodegradable oligomers andpolymers can also be suitably used.

As the modified bodies of naturally synthesized biodegradable oligomersand polymers, lignin and the like can be used. Lignin is adehydrogenated polymer of coniferyl alcohol and sinapyl alcoholcontained in a proportion of 20 to 30% in wood, and biodegraded.

Among the biodegradable resin materials as described above, artificiallysynthesized biodegradable oligomers and polymers and modified bodies ofartificially synthesized biodegradable oligomers and polymer havesuitable bonding force between molecules, resultantly, thermoplasticitythereof is excellent, the viscosity in melting does not increaseremarkably, and molding processability thereof is excellent, preferably.

Of them, polyesters and modified bodies of polyesters are preferable,and aliphatic polyesters and modified bodies of aliphatic polyesters arefurther preferable. Further, polyamino acids and modified bodies ofpolyamino acids are preferable, and aliphatic polyamino acids andmodified bodies of aliphatic polyamino acids are further preferable.Furthermore, polyols and modified bodies of polyols are preferable, andaliphatic polyols and modified bodies of aliphatic polyols are furtherpreferable.

The number-average molecular weight of a raw material biodegradableresin is, from the standpoint of the abilities of the resultingbiodegradable resin (processability, heat resistance of molded body,mechanical property of molded body, and the like), preferably 100 ormore and preferably 1000000 or less, more preferably 500000 or less,further preferably 100000 or less, most preferably 10000 or less.

By introducing a functional group forming a thermo-reversiblecross-linked structure into the biodegradable resin materials,derivatives thereof or modified bodies thereof, a thermo-reversiblecross-linkable biodegradable resin can be produced.

A functional group necessary for thermo-reversible cross-linking may beintroduced to the end of a molecule chain of a biodegradable resinmaterial or into a molecule chain. As the method for introduction,addition reaction, condensation reaction, copolymerization reaction andthe like can be used. Many biodegradable resin materials have functionalgroups such as a hydroxyl group, carboxyl group, amino group and thelike. Therefore, these functional groups can be directly utilized as athermo-reversible cross-linked portion, or these functional groups canbe derived into a functional group forming thermo-reversiblecross-linking.

For example, when a hydroxyl group is necessary, the following methodsare possible.

(i) Polysaccharides and polyols have already a hydroxyl group.

(ii) Polyesters have a hydroxyl group and/or carboxyl group at the endof a molecule chain. As polyesters carrying a hydroxyl group at bothends of a molecule chain, for example, both end-hydroxy PBS(polybutylene succinate) is mentioned. The both end-hydroxy PBS can beobtained by, for example, charging 1,4-butanediol and succinic acid sothat the 1,4-butanediol/succinic acid (molar ratio) is preferably morethan 1, more preferably 1.05 or more, further preferably 1.1 or more,and conducting a dehydration condensation reaction.

(iii) On the other hand, regarding polyesters having a carboxyl group atthe end of a molecule chain, the carboxyl group can be sealed with ahydroxyl group to obtain polyesters having a hydroxyl group at bothends. As the compound used for sealing, those having two or morehydroxyl groups such as diols and polyols are desirable, and it isparticularly desirable to use a compound having three or more hydroxylgroups since then a cross-linked point of three-dimensional cross-linkedstructure can be formed. For example, by esterifying a carboxyl group ofpolylactic acid obtained by ring-opening polymerization of lactide, withpentaerythritol to seal the carboxyl group, a polyester having ahydroxyl group at both ends of a molecule is obtained. “Sealing with ahydroxyl group” means that, for example, the end is derived into ahydroxyl group.

(iv) It is also possible to prepare a polyester having a hydroxyl groupat the end, by sequentially adding lactide by ring-openingpolymerization to a poly-functional hydroxyl compound as a core,according to a method of Chan-Ming D. et al. (Polymer, vol. 42, p. 6891,2001).

In an esterification reaction, reagents such as carbodiimides and thelike can also be used in addition to acids and alkalis. Esterificationcan also be performed by converting a carboxyl group into an acidchloride using thionyl chloride or allyl chloride, then, reacting thiswith a hydroxyl group. Regarding those synthesized by using as a rawmaterial a dicarboxylic acid and diol such as polybutylene succinate,polyethylene succinate and polybutylene succinate adipate, end groups ofa molecular chain can all be converted into a hydroxyl group by settinga molar ratio of diol/dicarboxylic acid of raw materials used of morethan 1.

By a transesterification reaction, the end group can be converted into ahydroxyl group. Namely, a polyester resin can be transesterified with acompound having two or more hydroxyl groups, to obtain a polyester resinhaving a hydroxyl group at the end.

It is particularly desirable to use a compound having three or morehydroxyl groups, as the compound having a hydroxyl group, since then across-linking point of a three-dimensional cross-linked structure can beformed. For example, by transesterifying an ester bond of polylacticacid with pentaerythritol, a polyester having 4 hydroxyl groups in totalat the end of a molecule chain is obtained. A resin having a carboxylicacid at the end part and a compound having an unreacted hydroxyl groupcan be easily purified and removed.

When a biodegradable resin material or a biodegradable resin materialmodified with a hydroxyl group is subjected to an esterificationreaction with hydroxybenzoic acid, a hydroxyl group can be convertedinto a phenolic hydroxyl group.

When a carboxyl group is necessary, if a compound having a 2 ormore-functional carboxylic acid is bonded to a hydroxyl group carried ona biodegradable resin material by the above-mentionedtransesterification reaction, it can be modified into a carboxyl group.Particularly when an acid anhydride is used, a biodegradable resinmaterial having a carboxyl group can be easily prepared. As the acidanhydride, pyromellitic anhydride, trimellitic anhydride, phthalicanhydride, hexahydrophthalic anhydride, maleic anhydride and derivativesthereof can be utilized.

(Chemical Structure of Cross-Linked Portion)

The cross-linked portion cosists of two groups, a first functional groupand a second functional group, manifesting cleaving by heating andcovalent bonding by cooling. When solidified at temperatures lower thanthe melt processing temperature, a first functional group and a secondfunctional group form cross-linking by a covalent bond, and at giventemperatures equal to or higher than the melt processing temperature andthe like, cleavage into a first functional group and a second functionalgroup occurs. The bonding reaction and cleaving reaction of across-linked portion progress reversibly with the change in temperature.A first functional group and a second functional group may be differentfunctional groups or the same functional group. When the same twofunctional groups are symmetrically bonded to form cross-linking, thesame functional group can be used as the first functional group and thesecond functional group.

The reversible reaction mode for bonding by heating to form across-linked portion and cleaving by cooling is not particularlyrestricted, and desirably selected from the following modes, from thestandpoint of the productivity of a resin substance, the moldingproperty of a resin substance, the abilities of a molded body(mechanical properties and heat resistance and the like) and the like.

(1) Diels-Alder Type Cross-Linking

A Diels-Alder [4+2] cycloaddition is utilized. By introducing aconjugated diene and dienophile as a functional group, a biodegradableresin forming thermo-reversible cross-linking is obtained. As theconjugated diene, for example, a furan ring, thiophene ring, pyrrolering, cyclopentadiene ring, 1,3-butadiene, thiophene-1-oxide ring,thiophene-1,1-dioxide ring, cyclopenta-2,4-dienone ring, 2H pyrane ring,cyclohexa-1,3-diene ring, 2H pyrane-1-oxide ring, 1,2-dihydropyridinering, 2H thiopyrane-1,1-dioxide ring, cyclohexa-2,4-dienone ring,pyran-2-one ring and the substituted bodies of them, and the like can beused as a functional group. As the dienophile, an unsaturated compoundreacting additively with a conjugated diene to give a cyclic compound isused. For example, a vinyl group, acetylene group, allyl group, diazogroup, nitro group and substituted bodies thereof are used as afunctional group. The above-mentioned conjugated diene also acts as adienophile, in some cases.

Of them, for example, cyclopentadiene can be used in a cross-linkingreaction. Cyclopentadiene has both the action as a conjugated diene andthe action as a dienophile. Dicyclopentadienedicarboxylic acid as adimer of cyclopentadiene carboxylic acid can be obtained easily fromcommercially available cyclopentadienyl sodium (E. Rukcenstein et al.,J. Polym. Sci. Part A: Polym. Chem., vol. 38, pp. 818 to 825, 2000).This dicyclopentadienedicarboxylic acid is introduced as a cross-linkingportion into a biodegradable resin material having a hydroxyl group or abiodegradable resin material modified with a hydroxyl group and thelike, at a site of the presence of a hydroxyl group, by anesterification reaction.

When 3-maleimidepropionic acid and 3-furylpropionic acid are, forexample, used, a cross-linking portion can be introduced easily into abiodegradable resin material having a hydroxyl group or a biodegradableresin material modified with a hydroxyl group and the like, at a site ofpresence of a hydroxyl group, by an esterification reaction.

In the above-mentioned esterification reaction utilized for introductionof a cross-linking portion, catalysts such as carbodimides and the likecan also be used in addition to acids and alkalis and the like. It isalso possible that a carboxyl group is derived into an acid chlorideusing thionyl chloride or allyl chloride and the like, then, this isreacted with a hydroxyl group to cause esterification. When an acidchloride is used, it can also be introduced at the side of an aminogroup of amino acids and derivatives thereof since it easily reacts alsowith an amino group.

These functional groups form a thermo-reversible cross-linked structureas shown in the following general formula (I).

(2) Nitroso Dimer Type Cross-Linking

Nitrosobenzene, for example, is used in a cross-linking reaction. As thenitrosobenzene, for example, dinitrosopropane, dinitrosohexane,dinitrosobenzene, dinitrosotoluene and the like are used. For example,by reacting a dimer of 4-nitroso-3,5-benzylic acid (a method ofsynthesizing a dimer of 4-nitroso-3,5-dichlorobenzoyl chloride isdescribed in U.S. Pat. No. 3,872,057) with a hydroxyl group of abiodegradable resin material having a hydroxyl group, a hydroxyl groupof a biodegradable resin material modified with a hydroxyl group, andthe like, a thermo-reversible cross-linked portion can be easilyintroduced into a portion of the presence of a hydroxyl group. When anacid anhydride is used, it can also be introduced at the side of anamino group of amino acids and derivatives thereof since it easilyreacts also with an amino group.

These functional groups form a thermo-reversible cross-linked structureas shown in the following general formula (II).

In the general formula (II), two nitroso groups form a nitroso dimer bycooling to give cross-linking. This cross-linked portion is cleaved byheating.

(3) Acid Anhydride Ester Type Cross-Linking

An acid anhydride and a hydroxyl group can be used in a cross-linkingreaction. As the acid anhydride, an aliphatic carboxylic anhydride andan aromatic carboxylic anhydride and the like are used. Though any ofcyclic acid anhydrides and acyclic acid anhydrides can be used, cyclicacid anhydrides are suitably used. Examples of the cyclic acid anhydrideinclude a maleic anhydride group, phthalic anhydric group, succinicanhydride group and glutaric anhydride group, and examples of theacyclic acid anhydride include an acetic anhydride group, propionicanhydride group and benzoic anhydride group. Of them, a maleic anhydridegroup, phthalic anhydric group, succinic anhydride group, glutaricanhydride group, pyromellitic anhydride group, trimellitic anhydridegroup, hexahydrophthalic anhydride group, acetic anhydride group,propionic anhydride group and benzoic anhydride group and substitutedbodies thereof and the like are preferable as the acid anhydridereacting with a hydroxyl group to form a cross-linked structure.

As the hydroxyl group, a hydroxyl group of a biodegradable resinmaterial having a hydroxyl group, and a hydroxyl group of abiodegradable resin material and the like containing a hydroxyl groupintroduced by various reactions, are used. Also, hydroxyl compounds suchas diols and polyols and the like may be used as a cross-linking agent.Further, diamines and polyamines can also be used as a cross-linkingagent. When those having two or more acid anhydrides such aspyromellitic anhydride are, for example, used as the acid anhydride,they can be used as a cross-linking agent for a biodegradable resinmaterial having a hydroxyl group, a biodegradable resin materialmodified with a hydroxyl group.

By copolymerizing maleic anhydride with an unsaturated compound by vinylpolymerization, a compound having two or more maleic anhydrides isobtained easily (JP-A Nos. 11-106578, 2000-34376). This can also be usedas a cross-linking agent for a biodegradable resin material having ahydroxyl group, a biodegradable resin material modified with a hydroxylgroup.

The acid anhydride and hydroxyl group as described above form athermo-reversible cross-linked structure as shown in the followinggeneral formula (III).

In the general formula (III), an acid anhydride group and a hydroxylgroup form an ester by cooling to give cross-linking. This cross-linkingis cleaved by heating.

(4) Halogen-Amine Type Cross-Linking

A thermo-reversible cross-linked portion can be formed from a polyamineand tetramethylhexadiamine and the like and an alkyl halide. Forexample, a halide can be obtained by ester-bonding a halide having acarboxyl group such as 4-bromomethylbenzoic acid to a biodegradableresin material having a hydroxyl group or a biodegradable resin materialmodified with a hydroxyl group, and the like. By adding, for example,tetramethylhexanediamine as a cross-linking agent to this, abiodegradable resin forming thermo-reversible cross-linking is obtained.

Examples of the alkyl halide group include an alkyl bromide, alkylchloride, phenyl bromide, phenyl chloride, benzyl bromide and benzylchloride.

As the amino group, tertiary amino groups are preferable, and examplesthereof include a dimethylamino group, diethylamino group anddiphenylamino group. Of them, a dimethylamino group is preferable. Thecombination of an alkyl halide with a tertiary amino group is notparticularly restricted, and a combination of benzyl bromide withdimethylamino group is exemplified.

These functional groups form a thermo-reversible cross-linked structureas shown in the following general formula (IV).

In the general formula (IV), an alkyl halide group and a tertiary aminegroup form a covalent bond of quaternary ammonium salt type, to givecross-linking. This cross-linking is cleaved by heating.

(5) Urethane Type Cross-Linking

A thermo-reversible cross-linked portion can be formed from anisocyanate and active hydrogen. For example, a multi-functionalisocyanate is used as a cross-linking agent, and is reacted with ahydroxyl group, amino group and phenolic hydroxyl group of biodegradableresin materials and derivatives thereof. A molecule having two or morefunctional groups selected from a hydroxyl group, amino group andphenolic hydroxyl group can also be added as a cross-linking agent.Further, a catalyst can also be added for setting the cleavingtemperature in a desired range. Further, dihydroxybenzene,dihydroxybiphenyl, phenol resin and the like can also be added as across-linking agent.

A multi-functional isocyanate is used as a cross-linking agent, and isreacted with a hydroxyl group, amino group and phenolic hydroxyl groupof biodegradable resin materials and derivatives thereof. Further,dihydroxybenzene, dihydroxybiphenyl, phenol resin and the like can alsobe added as a cross-linking agent. As the multi-functional isocyanate,there can be used tolylene diisocyanate (TDI) and polymerized bodythereof, 4,4′-diphenylmethane diisocyanate (MDI), hexamethylenediisocyanate (HMDI), 1,4-phenylene diisocyanate (DPDI), 1,3-phenylenediisocyanate, 4,4′,4″-triphenylmethane triisocyanate, xylylenediisocyanate and the like.

For controlling the cleaving temperature, organic compounds such as1,3-diacetoxytetrabutyl distanoxane and the like, amines, metal soapsand the like may also be used as a cleaving catalyst.

The above-mentioned functional groups form a thermo-reversiblecross-linked structure as shown in the following general formula (V).

In the general formula (V), a phenolic hydroxyl group and an isocyanategroup form a urethane by cooling to give cross-linking. Thiscross-linking is cleaved by heating.

(6) Azlactone-Hydroxyaryl Type Cross-Linking

Examples of the aryl group include a phenyl group, tolyl group, xylylgroup, biphenyl group, naphthyl group, anthryl group, phenanethryl groupand groups derived from these groups, and a phenolic hydroxyl groupbonding to these groups reacts with an azlactone structure contained ina group forming a cross-linked structure. As that having a phenolichydroxyl group, used are biodegradable resin materials having a phenolichydroxyl group, biodegradable resin materials modified withhydroxylphenols, and the like.

As the azlactone structure, multi-functional aziactones such as1,4-(4,4′-dimethylazlactyl)butane,poly(2-vinyl-4,4′-dimethylazalactone), bisazlactonebenzene,bisazlactonehexane and the like are preferable.

Bisaziactylbutane cross-linked by an azlactone-phenol reaction, and thelike can also be used, and these are described, for example, Engle etal., J. Macromol. Sci. Re. Macromol. Chem. Phys., vol. C33, no. 3, pp.239 to 257, 1993.

These functional groups form a thermo-reversible cross-linked structureas shown in the following general formula (VI).

In the general formula (VI), an azlactone group and a phenolic hydroxylgroup form a covalent bond by cooling to give cross-linking. Thiscross-linking is cleaved by heating.

(7) Carboxyl-Alkenyloxy Type Cross-Linking

As that having a carboxyl group, biodegradable resin materials having acarboxyl group, biodegradable resin materials modified with a carboxylgroup, and the like are used. The alkenyloxy structure includes vinylether, allyl ether and structures derived from these structures, andthose having two or more alkenyloxy structures can also be used.

Further, alkenyl ether derivatives such as bis[4-(vinyloxy)butyl]adipate and bis[4-(vinyloxy)butyl] succinate and the like can also beused as a cross-linking agent.

These functional groups form a thermo-reversible cross-linked structureas shown in the following general formula (VII).

In the general formula (VII), a carboxyl group and a vinyl ether groupform a hemiacetal ester by cooling to give cross-linking. Thiscross-linking is cleaved by heating (JP-A Nos. 11-35675, 60-179479).

(8) Cross-Linking Agent

As described above, a compound having in its molecule two or morefunctional groups capable of forming a thermo-reversible cross-linkedportion can be a cross-linking agent.

Examples of the cross-linking agent having an acid anhydride groupinclude bisphthalic anhydride compound, bissuccinic anhydride compound,bisglutaric anhydride compound and bismaleic anhydride compound.

Examples of the cross-linking agent having a hydroxyl group includeglycols such as ethylene glycol, diethylene glycol, triethylene glycoland the like; and alcohol compounds such as 1,4-butanediol,1,6-hexaneidol, 1,8-octanediol, 1,10-decanediol, trimethylolethane,trimethylolpropane, pentaerythritol and the like.

Examples of the cross-linking agent having a carboxyl group includeoxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid,phthalic acid, maleic acid and fumaric acid.

Examples of the cross-linking agent having a vinyl ether group includebis[4-(vinyloxy)butyl] adipate, bis[4-(vinyloxy)butyl] succinate,ethylene glycol divinyl ether, butanediol divinyl ether,2,2-vis[p-(2-vinyloxyethoxy)phenyl]propane.

Examples of the cross-linking agent having an alky halide group includeα,α′-dibromoxylene, α,α′-dichloroxylene, bisbromomethylbiphenyl,bischloromethylbiphenyl, bisbromodiphenylmethane,bischlorodiphenylmethane, bisbromomethylbenzophenone,bischloromethylbenzophenone, bisbromodiphenylpropane andbischlorodiphenylpropane.

Examples of the cross-linking agent having a tertiary amino groupinclude tetramethylethylenediamine, tetramethylhexanediamine andbisdimethylaminobenzene.

Examples of the cross-linking agent having a phenolic hydroxyl groupinclude dihydroxybenzene, dihydroxybiphenyl, resol type phenol resin andnovolak type phenol resin.

Examples of the cross-linking agent having an isocyanate group includearomatic diisocyanates such as 2,4-tolylene diisocyanate, 2,6-tolylenediisocyanate, 4,4′-diphenylmethane diisocyanate, 2,4′-diphenylmethanediisocyanate, p-phenylene diisocyanate and the like, aliphaticdiisocyanates such as hexamethylene diisocyanate and the like, alicyclicdiisocyanates such as isophorone diisocyanate and the like, andarylaliphatic diisocyanates such as xylylene diisocyanate and the like.

Examples of the cross-linking agent having an azlactone group includebisazlactonebutane, bisazlactonebenzene and bisazlactonehexane.

Examples of the cross-linking agent having a nitroso group includedinitrosopropane, dinitrosohexane, dinitrosobenzene anddinitrosotoluene.

(Selection of Cross-Linked Structure)

As the mode of a reversible reaction of bonding by heating to form across-linked portion and cleaving by cooling, a Diels-Alder type,nitroso dimer type, acid anhydride ester type, halogen-amine type,urethane type, azlactone-hydroxyaryl type and carboxyl-alkenyloxy typeand the like can be utilized as described above, however, it may beadvantageous to avoid a chemical reaction showing deterioration of themain chain of a biodegradable resin by thermal decomposition, hydrolysisand the like, in some cases. Particularly, it may be advantageous toavoid a reaction generating a free carboxylic acid, in forming across-linked structure such as of acid anhydride ester type. However,when biodegradation speed is desired to be increased, a reactiongenerating a carboxylic acid may rather be preferable, in some cases. Inthe case of a halogen-amine type reaction, there is a possibility ofgeneration of dioxins in burning since a halogen is contained. In anyway, it is necessary to effect the selection of the reaction typecarefully.

The cleaving temperature of a cross-linked portion is preferably over120° C. since sufficient cross-linking is formed at the use temperaturesof a molded body of 100° C. or less. On the other hand, temperatures of280° C. or less are preferable and temperatures of 250° C. or less aremore preferable since then melt processing can be conducted at suitabletemperatures, from the standpoint of thermal decomposition of abiodegradable resin substance.

More specifically, a biodegradable resin is provided having a functionalgroup forming a thermo-reversible cross-linked structure which iscovalently bonded at temperatures for use as a molded article andcleaved at temperatures over the glass transition temperature (Tg),temperatures over heat resistant temperature necessary for a resin, andtemperatures equal to or lower than the molding temperature.

Temperatures over the glass transition temperature (Tg) and temperaturesover heat resistant temperature necessary for a resin include, forexample, 120° C., and at 120° C. or higher, the degradation of abiodegradable resin is not promoted, the cross-linked structure can beselected from a Diels-Alder type, carboxyl-alkenyloxy type and the like,and a nitroso dimer type, urethane type, azlactone-hydroxyaryl type andthe like can also be applied.

In the case of the Diels-Alder type, the cleaving reaction ofdicyclopentane progresses at 150° C. or more and 250° C. or less,therefore, it is possible to impart high heat resistance and excellentmolding property to a biodegradable resin. Regarding this cleavingtemperature of a cross-linked portion, for example, Chujo et al.,macromolecules, vol. 23, pp. 2636-2641, 1990 introduces that thecleaving temperature of a cleaving reaction in a solution is 80° C., ina reaction of furan-maleimide. On the other hand, Stephen A. C et al.(J. P. S. Part A: Polym. Chem., vol. 30, p. 1775, (1992)) introduces theexistence of those showing the maximum cleaving reaction at 150° C. andthose showing the maximum cleaving reaction at 210° C., and describesthat the initiation temperature of a cleaving reaction variessignificantly depending on the method for introducing a functional groupsince the degree of steric hindrance varies depending on thisintroduction method. As the method of stabilizing a bonding portion andraising the cleaving temperature, there are a method in which anelectron attractive functional group is imparted to a maleimide ring,and a method in which an electron donative functional group is impartedto a furan ring. By this, the bonding reaction can be made easy, and across-linked portion having high cleaving temperature and havingexcellent heat resistance can also be obtained.

Since the cleaving reaction of nitroso dimer type cross-linkingprogresses at 110° C. or more and 150° C. or less, this reaction canprovide high heat resistance and excellent molding property for abiodegradable resin.

Since the cleaving reaction of urethane type cross-linking progresses at120° C. or more and 250° C. or less depending on the above-mentionedselection of a catalyst and control of addition amount, this reactioncan provide high heat resistance and excellent molding property for abiodegradable resin.

Since the cleaving reaction of azlactone-hydroxyaryl type cross-linkingprogresses at 100° C. or more and 200° C. or less, this reaction canprovide high heat resistance and excellent molding property for abiodegradable resin.

A free carboxylic acid is not present at normal temperature in a resincross-linked by carboxyl-alkenyloxy type cross-linking, consequently,the moisture resistance of a biodegradable resin is not deteriorated,therefore, such as resin is preferable. Since the cleaving reaction ofcarboxyl-alkenyloxy type cross-linking progresses at 100° C. or more and250° C. or less for a carboxyl group, this reaction can provide highheat resistance and excellent molding property for a biodegradableresin.

Of the above-mentioned types, the Diels-Alder type andcarboxyl-alkenyloxy type are preferable since deterioration of abiodegradable resin is little and moisture resistance thereof is high,and preferable as the functional group are a hydroxyl group, carboxylgroup, alkenyl group, alkenyloxy group, and a group having a conjugateddouble bond.

From the standpoint of heat resistance, preferable as the cross-linkedstructure is a three-dimensional cross-linked structure.

The cross-linked density of a three-dimensional cross-linked structureis set at a desired value by controlling the number of functional groupsof a biodegradable resin and the mixing ratio of members and the like atgiven values. The cross-linked density of a three-dimensionalcross-linked structure is represented by the mol number of cross-linkingpoints of a three-dimensional structure contained per 100 g of a resinsubstance, and for realizing sufficient heat resistance, preferably0.0001 or more, more preferably 0.001 or more, further preferably 0.002or more, on the other hand, when the mol number of cross-linkingportions is over 10, the number of portions forming a biodegradableresin increases larger than the number of portions forming cross-linkedportions, and the viscosity in molding lowers, resultantly, an excellentmolded article cannot be obtained. The mol number is preferably 1 orless, further preferably 0.2 or less for realizing a recycling propertyand biodegradability, since biodegradability is not shown outside thisrange.

(Structure of Cross-Linked Body)

At least any one of the functional groups as described above iscontained in a first biodegradable resin, and in some cases, two or morefirst functional groups and two or more second functional groups arecontained in a first biodegradable resin.

A first functional group is present at the end of a molecule of a firstbiodegradable resin in some cases and present at parts other than theend such as a side chain and the like in some cases. For example, when afirst functional group is a hydroxyl group, polybutylene succinatehaving a hydroxyl group at both ends is an example of a firstbiodegradable resin having a first functional group present at the end.In this case, a first functional group is present on both ends of afirst biodegradable resin, however, in some cases, a first functionalgroup is present only at one end.

When a first functional group is a hydroxyl group, amylose and cellulosemethylated at both ends are examples of a first biodegradable resinhaving a first functional group at parts other than the end.

The main chain of a first biodegradable resin may be linear or branched,for example, an ester body in which 4 mols of polylactic acid areradially bonded around the center composed of 1 mol of pentaerythritolis an example of a branched first biodegradable resin. When a firstfunctional group is present at the end, a functional group is present onall ends in some cases, and a first functional group is present only atsome ends in some cases.

Further, there is also a case in which a plurality of first functionalgroups are bonded to the same site in the molecule chain of a firstbiodegradable resin, and for example, when pentaerythritol isester-bonded to the end of a carboxyl group of polylactic acid, this isan example in which three hydroxyl groups are bonded to the end of acarboxyl group of polylactic acid. In this case, carbon derived fromcentral methane of pentaerythritol is situated at the same site, and ahydroxyl group as a first functional group is bonded to this carbon viamethylene. That a plurality of first functional groups are bonded to thesame site means that a plurality of first functional groups are bondedvia 0 to 5 atoms counted from one atom, and from the standpoint of theabilities of the resulting thermo-reversible cross-linkablebiodegradable resin, it is preferable that a plurality of firstfunctional groups are bonded via 0 to 3 atoms.

From the standpoint of the productivity of a resin substance and themolding property of a resin substance, and the like, a firstbiodegradable resin having a first functional group present at the endof the molecule chain is preferable. In this case, since there existssuitable mutual action between first functional groups on differentmolecule chains in melt processing, excellent flowability andprocessability can be realized. From the standpoint of the abilities ofa molded body (mechanical property, heat resistance and the like),preferable are a branched first biodegradable resin and a firstbiodegradable resin in which a plurality of first functional groups arebonded to the same site. In this case, since three-dimensionalcross-linking is formed in a molded body, a molded body having excellentmechanical property and heat resistance can be obtained.

When there are two or more covalent-bondable functional groups, thereare also a case in which other functional group (second functionalgroup) is present in a biodegradable resin (first biodegradable resin)containing one functional group (first functional group) presenttherein, and a case in which a second functional group is present in abiodegradable resin (second biodegradable resin) other than abiodegradable resin (first biodegradable resin) containing a firstfunctional group present therein. Examples in which both a firstfunctional group and a second functional group are present in the samefirst biodegradable resin will be listed below.

(1) A multi-functional carboxylic acid resin in which a hydroxyl groupof amylose and cellulose forms an ester bond with maleic anhydride isprepared. To part of carboxylic acids in this resin, 2-aminoethyl vinylether is ester-bonded via carbodiimides. In this case, a carboxylic acidstructure (first functional group) and a vinyl ether group (secondfunctional group) are present in the same biodegradable resin (firstbiodegradable resin), to form cross-linking of a carboxyl-alkenyloxytype.

(2) In the case of a first biodegradable resin in which a Diels-Alderreaction production of cyclopentadiene carboxylic acid and maleimide(3,5-dioxo-4-aza-tricyclo[5.2.1.02,6]deca-8-ene-10-carboxylic acid) isfurther ester-bonded four hydroxyl groups on both ends in a compoundcontaining pentaerythritol ester-bonded to the end of a carboxyl groupof polylactic acid, a first functional group and a second functionalgroup are the identical cyclopentadiene derivative, a first functionalgroup and a second functional group are presents in the same firstbiodegradable resin, and by removing maleimide by heating under reducedpressure, Diels-Alder type cross-linking composed of cyclopentadienes isformed. Cross-linking is formed at both ends of a first biodegradableresin.

(3) In the case of a first biodegradable resin in which cyclopentadienecarboxylic acid is ester-bonded to both ends of polybutylene succinatehaving a hydroxyl group at both ends, a first functional group and asecond functional group are the identical cyclopenta-2,4-dien-1-ylgroup, a first functional group and a second functional group arepresents in the same first biodegradable resin, and Diels-Alder typecross-linking is formed. Cross-linking is formed at both ends of themolecule chain of a first biodegradable resin.

(4) In the case of a first biodegradable resin in which nitrosobenzoicacid is ester-bonded to both ends of polybutylene succinate having ahydroxyl group at both ends, a first functional group and a secondfunctional group are the identical nitrosobenzoyl group, a firstfunctional group and a second functional group are presents in the samefirst biodegradable resin, and nitroso dimer type cross-linking isformed. Cross-linking is formed at both ends of a first biodegradableresin.

The above-mentioned resin substances (1) and (2) are obtained byintroducing a first functional group and a second functional group intoa first biodegradable resin material.

In producing the above-mentioned resin substances (3) and (4), acompound in which a first functional group and a second functional groupforming a cross-linked portion are previously covalently bonded, andhaving a group reacting with a first biodegradable resin material inaddition to the first functional group and second functional group (forexample, dimer of dicyclopentadienedicarboxylic acid, nitrosobenzoicacid, and the like) can be used as a cross-linking agent. When such across-linking agent and a first biodegradable resin material are mixedand reacted to allow the cross-linking agent to bond to the firstbiodegradable resin material, a resin substance having a portion undercross-linked condition can be obtained with good productivity.Particularly, when a first functional group and a second functionalgroup are identical and the identical functional groups symmetricallybond to form a cross-linked portion as in the above-mentioned (3) and(4), a dimer containing functional groups symmetrically bonded such asdicyclopentadienedicarboxylic acid, dimer of nitrosobenzoic acid and thelike can be used as a cross-linking agent.

When a cross-linking agent contains a plurality of functional groups, ifthe functional groups are identical, production of a cross-linking agentis easy, and a cross-linking reaction can be easily controlled,desirably.

On the other hand, a second functional group may also be present in asecond biodegradable resin other than a first biodegradable resincontaining a first functional group present therein. As an example ofsuch a case, there is mentioned a combination of a first biodegradableresin in which 3-maleimidepropionic acid is further ester-bonded to 4hydroxyl groups on both ends in a compound containing pentaerythritolester-bonded to the end of a carboxyl group of polylactic acid, with asecond biodegradable resin in which 3-furylpropionic acid is furtherester-bonded to 4 hydroxyl groups on both ends in a compound containingpentaerythritol ester-bonded to the end of a carboxyl group ofpolylactic acid. The first functional group has a maleimide structure,the second functional group has a furyl structure, and these functionalgroups are cross-linked in Diels-Alder type. Cross-linking is formed ofthe molecule end of a first biodegradable resin and the molecule end ofa second biodegradable resin.

Further, a resin substance can also be constituted of a mixturecontaining a first biodegradable resin having both a first functionalgroup and a second functional group, a second biodegradable resin havingboth a first functional group and a second functional group, a firstbiodegradable resin having only one of a first functional group and asecond functional group, a second biodegradable resin having only one ofa first functional group and a second functional group, and the like.

Also in producing such a resin substance, a compound in which a firstfunctional group and a second functional group forming a cross-linkedportion are previously covalently bonded, and having a group reactingwith a first biodegradable resin material in addition to the firstfunctional group and second functional group can be used as across-linking agent. When such a cross-linking agent and a firstbiodegradable resin material and a second biodegradable resin materialare mixed and reacted to allow the cross-linking agent to bond to thefirst biodegradable resin material and second biodegradable resinmaterial, a resin substance having a portion under cross-linkedcondition can be obtained with good productivity.

On the other hand, a second functional group is present in a linker, insome cases. In this case, a resin substance is constituted, at least, ofa first biodegradable resin having a first functional group and a linkerhaving a second functional group, and as the linker, that notdeteriorating the biodegradability of a first biodegradable resin isused. By using a linker, wider resin substances can be obtained,therefore, degrees of freedom of the productivity of a resin substance,the molding property of a resin substance, the abilities of a moldedbody (mechanical property, heat resistance and the like) and the likeare enlarged.

The linker includes monomers, oligomers, polymers and the like havingtwo or more second functional groups in one molecule, and the secondfunctional group in the linker forms a cross-linked portion with a firstfunctional group in a first biodegradable resin. The linker may also usemonomers, oligomers, polymers and the like having two or more firstfunctional groups in one molecule, in combination. Resultantly, in amolded body, two or more first biodegradable resins are cross-linked viaone or more linkers. In melting, a cross-linked portion is cleaved, andthe bonding and cleaving of a cross-linked portion are in a relation ofa thermo-reversible reaction. A linker having two or more secondfunctional groups in one molecule may be called a cross-linking agent insome cases, and such a linker and a first biodegradable resin are mixedand reacted, to produce a resin substance. If necessary, a plurality oflinker is used in combination in some cases, and a plurality of firstbiodegradable resins is used in combination in some cases.

As described above, the method (1) in which a functional group forcross-linking is introduced into a biodegradable resin, and the method(2) in which a linker is also used have been explained, and as othermethods, a method in which a system for mutually cross-linking linkersis introduced in a usual biodegradable resin can also be used. Forexample, a resin which is polymerized by a Diels-Alder reaction ispartially mixed in a commercially available biodegradable resin. As theresin to be subjected to a Diels-Alder reaction, the linkers describedabove can be used.

As monomeric linkers, the following compounds are exemplified.

(1) Toluene diisocyanate is used as a linker. In this case, a secondfunctional group is an isocyanate group, and as the first biodegradableresin, for example, biodegradable polyesters having a phenolic hydroxylgroup are used. The first functional group is a phenolic hydroxyl group,and an isocyanate group of toluene diisocyanate forms cross-linking witha phenolic hydroxyl group of a biodegradable polyester by urethane bond,and the biodegradable polyester having a phenolic hydroxyl group iscross-linked via toluene diisocyanate.

(2) N,N′-bismaleimide-4,4′-diphenylmethane is used as a linker. In thiscase, a second functional group has a maleimide structure, and as thefirst biodegradable resin, for example, polylactic acid in which froicacid is ester-bonded to the end of a hydroxyl group is used. The firstfunctional group is a furyl group, and the maleimide structure ofN,N′-bismaleimide-4,4′-diphenylmethane forms cross-linking ofDiels-Alder type with a furyl group bonded to polylactic acid, andpolylactic acid is cross-linked at one end viaN,N′-bismaleimide-4,4′-diphenylmethane.

When a linker contains a plurality of functional groups, if thefunctional groups are identical, production of a linker is easy, and across-linking reaction can be easily controlled, desirably.

(Use of Electrostatically Bondable Cross-Linked Structure inCombination)

Electrostatic bond is what is bonded electrostatically and means a bondformed by electrostatic attractive force, and includes ionic bond andhydrogen bond and the like. These bonds include a case of directformation by a functional group and a functional group, a case offormation by a functional group and a functional group via an ion, acase of formation by a functional group and a functional group via apolyion, and the like.

As the electrostatic bond directly formed by a functional group and afunctional group, a case of formation between ion pair between ionicfunctional groups is mentioned. As the electrostatic bond formed by afunctional group and a functional group via an ion, a case in which twoor more ionic functional groups are coordinated to one counter ion byelectrostatic attractive force is mentioned. Further, as theelectrostatic bond formed by a functional group and a functional groupvia a polyion, a case in which two or more ionic functional groups arecoordinated to one ionic polymer by electrostatic attractive force ismentioned.

A biodegradable resin obtained from a biodegradable resin material has afunctional group, and the electrostatic bond mode includes a case inwhich a functional group forms an ion pair, a case in which a functionalgroup is coordinated to a counter ion by electrostatic attractive force,a case in which a functional group is coordinated to a polyion byelectrostatic attractive force, and the like.

The embodiment in which a functional group forms an ion pair is anexample of direct formation of electrostatic bond between a functionalgroup and a functional group, and for example, there is a case in whicha carboxyl group in a biodegradable resin becomes a carboxylate anion,an amino group in a biodegradable resin becomes an ammonium cation, andthese form an ion pair to give an organic salt, or the like.

The embodiment in which a functional group is coordinated to a counterion by electrostatic attractive force is an example of formation ofelectrostatic bond via an ion between a functional group and afunctional group, and for example, there is a case in which two or morecarboxyl groups in a biodegradable resin are ion-bonded to one metalcation, or the like.

Further, the embodiment in which a functional group is coordinated to apolyion by electrostatic attractive force is an example of formation ofelectrostatic bond between a functional group and a functional group viaa polyion, and for example, there are a case in which two or morecarboxyl groups in a biodegradable resin are ion-bonded to onepolycation such as pentaethylenehexamine and polyamine, a case in whichtwo or more amino groups in a biodegradable resin are ion-bonded to onepolyanion such as benzenetricarboxylic acid and polyacrylic acid. As thepolyion, monomers having one or more, preferably two or more ionicfunctional groups; oligomers having one or more, preferably two or moreionic functional groups; polymers having one or more, preferably two ormore ionic functional groups, and the like can be used.

The ionic functional group is a functional group decomposed into an ionor bonded with an ion to become itself an ion. The electrostaticallybondable cross-linked structure formed from an ionic functional groupcan be formed from a cationic functional group and an anionic functionalgroup utilizing electrostatic bond. As the cationic functional group, anamino group, imino group and the like are used. As the anionicfunctional group, a carboxyl group, sulfonyl group, phosphoric group,groups containing a halide ion, hydroxyl group, phenolic hydroxyl group,thiocarboxyl group and the like are used. A cross-linked structure withelectrostatically bonding can be formed also by using a molecule havingone or more ionic functional groups such as an alkali metal ion,alkaline earth metal ion, transition metal ion, anion, polycation,polyanion and the like, instead of the cationic functional groups andanionic functional groups.

Specific examples of the electrostatic bond type will be illustratedbelow.

(1) Bonding Via Ion

A cross-linked structure by electrostatic bond via an ion is called ioncross-linking, and in the case of ion cross-linking, for example, abiodegradable resin material having an anionic functional group such asa carboxyl group and the like is used, or that which is obtained byintroducing an anionic functional group such as a carboxyl group and thelike into a biodegradable resin material is used. A cationic functionalgroup can be introduced as a counter ion of an anionic functional groupby neutralizing the biodegradable resin material having a carboxyl groupas described above using a halide, inorganic acid salt, sulfate,nitrate, phosphate, organic acid salt, carboxylate or the like as thecationic functional group. For the neutralization treatment, one or moresalts selected from the above-mentioned salts may be added directly oradded in the form of aqueous solution to a biodegradable resin materialin melted condition. Further, after dissolution of a biodegradable resinmaterial in water and/or organic solvent, one or more salts selectedfrom the above-mentioned salts may also be added.

The form of thus obtained biodegradable resin includes a structure inwhich two or more cations are electrostatically bonded via one anion, astructure in which two or more anions are electrostatically bonded viaone cation, and the like.

The ion used for ion cross-linking includes an alkali metal ion,alkaline earth metal ion, transition metal ion, organoammonium, halideion, carboxylate anion, alcoholate anion, phenolate anion,thiocarboxylate anion, sulfonate anion and the like, and if necessary,two or more ions can also be used in combination.

Of these ions, di- or more-valent ions are preferable from thestandpoint of heat resistance.

From the standpoint of the abilities (mechanical property, heatresistance and the like) of the resulting resin substance and moldedbody, a combination of a biodegradable resin having a carboxyl group anda metal ion is preferable, and as the metal ion, preferable are a sodiumion, calcium ion, zinc ion, magnesium ion, copper ion and the like. Ifnecessary, two or more metal ions can be also used in combination.

The neutralization ratio of a carboxyl group is preferably 1% or more,more preferably 5% or more, further preferably 10% or more, and mostpreferably 15% or more. The neutralization ratio of a carboxyl group is100% or less, preferably 95% or less.

In the case of thus obtained biodegradable resin, a structure isobtained in which two or more carboxyl groups are electrostaticallybonded via a metal ion.

(2) Bonding Via Polyion

The cross-linked structure by electrostatic bond via a polyion is calledpolyion cross-linking, and as polycation monomers having one or more,preferably two or more ionic functional groups among polyions used forpolyion cross-linking, tetraethylenepentamine, hexanediamine,2,4,6-triaminotoluene and the like can be used in addition topentaethylenehexamine.

As polyanion monomers having one or more, preferably two or more ionicfunctional groups, 2,3-dimethylbutane-1,2,3-tricarboxylic acid and thelike can be used in addition to benzenetricarboxylic acid.

As polycation oligomers and polymers having one or more, preferably twoor more ionic functional groups, polyamines such as polyvinyiamine,polyethyleneimine and the like can be used in addition to polyamines.

As polyanion oligomers and polymers having one or more, preferably twoor more ionic functional groups, polystyrenesulfonic acid,polyphosphoric acid and the like can be used in addition to polyacrylicacid.

(3) Bond by Formation of Organic Salt

A cross-linked portion can be formed, for example, by using a bondformed electrostatically between a cationic functional group such as anamino group and the like, and an anionic functional group such as acarboxylic acid and the like.

(Molding Processing)

In producing a molded body using a biodegradable resin substance formingthermo-reversible cross-linking obtained as described above, inorganicfillers, organic fillers, reinforcing materials, coloring agents,stabilizers (radical scavenger, antioxidant and the like), antibacterialagents, antifungal agents, flame retardants and the like can be used, ifnecessary.

As the inorganic filler, silica, alumina, talc, sand, clay, slag and thelike can be used. As the organic filler, organic fibers such asvegetable fiber and the like can be used. As the reinforcing material,glass fiber, carbon fiber, needle-like inorganic substances, fibrousTeflon resin and the like can be used. As the antibacterial agent, asilver ion, copper ion, zeolite containing them, and the like can beused. As the flame retardant, a silicone-based flame retardant,bromine-based flame retardant, phosphorus-based flame retardant, and thelike can be used.

The cleaving temperature of a cross-linked portion should be over 120°C. since sufficient cross-linking is formed at use temperatures of amolded body, on the other hand, the cleaving temperature is preferably280° C. or less, more preferably 250° C. or less since melt processingcan be conducted at temperatures causing no problem of thermaldecomposition of a biodegradable resin substance. After melting, abiodegradable resin substance is cooled and shaped. The coolingtemperature is preferably 0° C. or more, more preferably 10° C. or more,for formation of sufficient cross-linking, while preferably 100° C. orless, more preferably 80° C. or less. During a cooling process and aftera cooling process, a molded body is kept at given temperature in somecases, if necessary, for forming sufficient cross-linking andmanifesting the sufficient property of a molded body. By keeping thetemperature of a molded body, formation of cross-linking furtherprogresses, and the property of a molded body can be improved.

From the analogous standpoint, also the melting temperature (flowinitiation temperature) of a biodegradable resin substance should beover 120° C., while preferably 280° C. or less, more preferably 250° C.or less.

The resin and resin composition as described above can be processed intoa molded body for electric and electronic appliance use such as a casingof an electric article, construction material use, automobile part use,daily good use, medical use, agriculture use and the like, by a methodsuch as an injection molding method, film molding method, blow moldingmethod, foaming molding method and the like.

The thermo-reversible cross-linked structure can be utilized in a shapememory resin. As the shape memory resin, there is a resin described inIrie Masahiro et al., Keijo Kioku Polymer no Zairyo Kaihatsu(development of material of shape memory polymer) (ISBN 4-88231-064-3).The shape memory phenomenon generally means a phenomenon in which when amaterial, after deformation-processed at given temperatures, is heatedagain, the original shape is recovered. Namely, deformation-processingis conducted at temperatures equal to or higher than the glasstransition temperature of a resin, and deformation is fixed by coolingto temperatures equal to or lower than the glass transition temperature.(For use of a resin fixed at normal temperatures, it is necessary thatthe glass transition temperature is higher than normal temperatures) Forrecovery of shape, fixation of deformation by glass condition isreleased, by heating a resin at temperatures equal to or higher than theglass transition temperature. Here, it is possible to use athermo-reversible cross-linked structure, as a method for fixing thisdeformation of a resin. When a thermo-reversible cross-linked structureis used for fixation of deformation, it is possible to recover theoriginal shape at temperatures equal to or higher than the glasstransition temperature, by setting the cross-linking cleavingtemperature at the glass transition temperature or less. U.S. Pat. No.5,043,396 is an example of this method. It is also possible to use athermo-reversible cross-linked structure as a fixation point for memoryof shape. A shape memory resin needs a fixation point (or freezingphase) for preventing flow of a resin (creep phenomenon). That usingmutual entangling of polymers is called thermoplastic shape memoryresin, and by melting, it can be recycled. However, the shape recoveryforce is weak, and the recovery speed is also slow. In contrast, thatusing a covalent bond in a fixation point is called thermosetting shapememory resin, and it cannot be melted and consequently cannot berecycled while it has strong shape recovery force and also has highrecovery speed. When a thermo-reversible cross-linked structure is usedas this fixation point, a shape memory resin showing strong recoveryforce and high recovery speed and which can be melted and can berecycled can be obtained.

From the above-described facts, among biodegradable resins, preferableare polyester-based resins, and for example, polylactic acid ispreferable and polybutylene succinate is also preferable. As thecross-linked portion to be introduced in these biodegradable resins,Diels-Alder type cross-linking or carboxyl-alkenyloxy type cross-linkingis preferable. When Diels-Alder type cross-linking orcarboxyl-alkenyloxy type cross-linking is thus introduced into apolyester-based biodegradable resin such as polylactic acid and thelike, it is particularly preferable that the cross-linked portion has athree-dimensional cross-linked point and it is preferable that thecross-linked density of a three-dimensional cross-linked point is 0.0025to 0.110. The cleaving temperature of a cross-linked portion ispreferably 120° C. or more.

By selecting the chemical structures as described above, heat resistancecan be improved, the sufficient material recycling property of abiodegradable resin can be realized, an excellent molding property canbe realized, and durability such as moisture resistance and the like canbe made sufficient, without losing biodegradability.

The reason for this can be hypothesized as described below.

(i) Artificially synthesized biodegradable resins typically includingpolylactic acid and polybutylene succinate are, in general, excellent inmolding property as compared with polysaccharides of naturallysynthesized biodegradable resins.

(ii) Artificially synthesized biodegradable resins typically includingpolylactic acid and polybutylene succinate are, in general, excellent inmass production property as compared with resins synthesized by amicroorganism of naturally synthesized biodegradable resins.

(iii) Since polylactic acid can use lactic acid, a vegetable-derived rawmaterial, among artificially synthesized biodegradable resins, theconsumption of fossil fuels can be suppressed, and the generation amountof CO₂ can be suppressed.

(iv) Diels-Alder type cross-linking and carboxyl-alkenyloxy typecross-linking are not ionic at use temperatures of 100° C. or less,therefore, they do not promote the hydrolysis of the main chain ofpolylactic acid and polybutylene succinate. When used in applications ofdurable materials such as a casing of an electronic appliance and thelike, durability (moisture resistance) is required, and theabove-mentioned biodegradable resins can be suitably used also in suchapplications.

(v) By introducing a three-dimensional cross-linked point, a resinformed of a cross-linked substance has a three-dimensional structure,consequently, heat resistance is manifested. Due to the presence ofcross-linked points in sufficient amount, heat resistance can beremarkably improved. On the other hand, when cross-linked density is toohigh, the proportion of reversibly cross-linked portions occupying abiodegradable resin increases, consequently, the function as abiodegradable resin may be deficient in some cases.

(vi) When the cleaving temperature of a cross-linked portion is 120° C.or more, the heat resistance of a biodegradable resin can be 100° C. ormore.

(vii) When the cleaving temperature is 250° C. or less, molding ispossible without causing thermal decomposition of the main chain of abiodegradable resin.

The present invention will be illustrated further in detail by examplesbelow, however, they do not limit the scope of the invention. Unlessotherwise stated, reagents and the like used are commercially availablehigh purity products. The number-average molecular weight andweight-average molecular weight were measured by a gel permeationchromatogram method, and converted using standard polystyrene.

Abilities were evaluated by the following method.

Heat resistance: The penetration test (according to JIS K 7196, load 0.2g, needle diameter 3 mm) was conducted using a TMA measuring apparatusmanufactured by Shimadzu Corp. (trade name: TMA-40), and under 100° C.or less, that having deformation is represented by x, that havingsubstantially no deformation is represented by ◯, and that havingutterly no deformation is represented by ⊚. A specimen was kept at 100°C. for 2 hours before measurement.

Cleaving temperature: Using a DSC measuring apparatus manufactured bySeiko Instruments (trade name: DSC 6000), measurement was conducted at atemperature raising rate of 10° C./min, and the endothermic peak wasused as the cleaving temperature.

Biodegradability: A molded body (thickness: 0.1 mm) was produced by aheat press (200° C.) and buried in soil, and that showing degradationafter 6 months is represented by ◯, and that showing no degradationafter 6 months is represented by x.

Recycling property: A specimen was heated up to 200° C. to give meltcondition and subsequently cooled to normal temperature, and this cyclewas repeated 5 times (5 cycles of 200° C. and normal temperature), then,the above-mentioned heat resistance test was conducted, and under 100°C. or less, that showing deformation is represented by x, and thatshowing no deformation is represented by ◯.

Molding property: A specimen of 6.4 mm×12.5 mm×125 mm was injectionmolded at 200° C., and that which could be molded is represented by ◯,and that which could not be molded is represented by x.

Moisture resistance: A specimen was left for 6 months under conditionsof 20° C. and 60% RH, then, dried under reduced pressure at 80° C. Theviscosity of a resin substance at the molding temperature was measured,and compared with the viscosity before the moisture resistance test.

EXAMPLE 1-1

Into a 3 L separable flask equipped with a stirrer, fractionalcondenser, thermometer and nitrogen introducing tube were charged 716 g(6.1 mol) of succinic acid and 613 g (6.8 mol) of 1,4-butanediol, anddehydration condensation was conducted under a nitrogen atmosphere at180 to 220° C. for 3 hours. Subsequently, a de-glycol reaction wasconducted under reduced pressure at 180 to 220° C. for 3 hours, andwater and vinyl glycol were distilled off, to obtain a both end-hydroxylgroup aliphatic polyester (A1) having a number-average molecular weightof 3000.

100 parts by weight of thus obtained both end-hydroxyl group aliphaticpolyester (A1) and 6.6 parts by weight of 1,2,3,4-butanetetracarboxylicdianhydride manufactured by New Japan Chemical Co., Ltd. (trade name:Rikacid BT-100, referred to also as compound (B1)) were melt-kneaded at200° C. by Mini Max Mix Truder (trade name) manufactured by Toyo SeikiK.K., to obtain a composition (1).

EXAMPLE 1-2

100 parts by weight of the both end-hydroxyl group aliphatic polyester(A1) and 7.3 parts by weight of pyromellitic anhydride were melt-kneadedat 200° C. by Mini Max Mix Truder (trade name) manufactured by ToyoSeiki K.K., to obtain a composition (2).

EXAMPLE 1-3

A composition (3) was obtained in the same manner as in the case of thecomposition (1), except that 10.4 parts by weight of methyl vinylether-maleic anhydride copolymer (B2) (number-average molecular weight:900000) was used instead of the compound (B1) (the number ofcross-linked points capable of forming three-dimensional cross-linkingis about 0.060 per 100 g of a resin substance).

The abilities of the composition obtained above were evaluated and theresults are shown in Table 1.

TABLE 1 Evaluation result heat recycling molding resistancebiodegradability property property composition 1 ◯ ◯ ◯ ◯ composition 2 ◯◯ ◯ ◯ composition 3 ◯ ◯ ◯ ◯ polyester A1 X ◯ ◯ ◯

From Table 1, it was found that the compositions (1) to (3) areexcellent in all abilities of heat resistance, biodegradability,recycling property and molding property.

Polyester Resin M-1 to M-10

(M-1) Both end-hydroxy PBS (polybutylene succinate):1,4-butanediol andsuccinic acid are charged so that 1,4-butanediol/succinic acid (molarratio) is more than 1, more preferably 1.05 or more, further preferably1.1 or more, and a dehydration condensation reaction thereof isconducted, to obtain both end-hydroxyl group PBS having a number-averagemolecular weight of 100 to 1000000. By reducing pressure at reactiontemperatures of 110 to 250° C., the dehydration condensation reactionprogresses to increase the molecular weight. Also by adding a catalystsuch as tetraisopropoxytitanium and the like in an amount of 0.1 to 5parts by weight per 100 parts by weight of the monomer mixture, thedehydration condensation reaction progresses to increase the molecularweight.

(M-2) PLA (polylactic acid):lactide (dimer of lactic acid) isring-opening-polymerized, to obtain polylactic acid having anumber-average molecular weight of 100 to 1000000. By setting thereaction temperature at 120 to 220° C., the ring-opening reactionprogresses. Also by using stannous octanoate as a catalyst in an amountof 0.01 to 1 part by weight per 100 parts by weight the monomers, thedehydration condensation reaction can be further progressed to increasethe molecular weight.

(M-3) End-hydroxy PLA:PLA (M-2) and pentaerythritol, by ester-bondingend-hydroxy PLA (M-3) having a number-average molecular weight of 100 to1000000 is obtained. By using pyridine and1-ethyl-3-(3′-dimethylaminopropyl)carbodiimide hydrochloride asdehydration catalysts in equimolar amounts in a chloroform solvent, theesterification reaction can be progressed. Also, it can be purified bywashing with water.

(M-4) Both end-phenolic hydroxy PBS: by ester-bonding both end-hydroxyPBS (M-1) and hydroxybenzoic acid, both end-phenolic hydroxy PBS (M-4)having a number-average molecular weight of 100 to 1000000 is obtained.By using pyridine and 1-ethyl-3-(3′-dimethylaminopropyl)carbodiimidehydrochloride as dehydration catalysts in equimolar amounts in achloroform solvent, the esterification reaction can be progressed.

(M-5) One end-phenolic hydroxy PLA: by ester-bonding PLA (M-2) andhydroxybenzoic acid, one end-phenolic hydroxy PLA (M-5) having anumber-average molecular weight of 100 to 100000 is obtained. By usingpyridine and 1-ethyl-3-(3′-dimethylaminopropyl)carbodiimidehydrochloride as dehydration catalysts in equimolar amounts in achloroform solvent, the esterification reaction can be progressed. Also,it can be purified by washing with water.

(M-6) End-phenolic hydroxy PLA: by ester-bonding end-hydroxy PLA (M-3)and hydroxybenzoic acid, end-phenolic hydroxy PLA (M-6) having anumber-average molecular weight of 100 to 100000 is obtained. By usingpyridine and 1-ethyl-3-(3′-dimethylaminopropyl)carbodiimidehydrochloride as dehydration catalysts in equimolar amounts in achloroform solvent, the esterification reaction can be progressed. Also,it can be purified by washing with water.

(M-7) Both end-carboxylic acid PBS:1,4-butanediol and succinic acid arecharged so that 1,4-butanediol/succinic acid (molar ratio) is preferablyless than 1, more preferably 0.95 or less, further preferably 0.9 orless, and a dehydration condensation reaction thereof is conducted, toobtain both end-hydroxyl group PBS having a number-average molecularweight of 100 to 1000000. By reducing pressure at reaction temperaturesof 110 to 250° C., the dehydration condensation reaction progresses toincrease the molecular weight. Also by adding a catalyst such astetraisopropoxytitanium and the like in an amount of 0.1 to 5 parts byweight to 100 parts by weight of the monomer mixture, the dehydrationcondensation reaction progresses to increase the molecular weight.

(M-8) Multi-functional carboxylic acid PBS: by ester-bonding bothend-hydroxy PBS (M-1) and pyromellitic acid, multi-functional carboxylicacid PBS (M-8) having a number-average molecular weight of 100 to1000000 is obtained. By using pyromellitic acid in large excess amount(10 to 100-fold mol) for a hydroxyl group, PBS in which pyromelliticacid is ester-bonded to both ends thereof is obtained. By refluxing bothend-hydroxy PBS (M-1) and pyromellitic acid in a mixed solvent ofchloroform and THF, or a toluene solvent, the esterification reactioncan be progressed. Pyromellitic acid used excessively can be removed bywashing with hot water after removal of the solvent.

(M-9) Multi-functional carboxylic acid PLA: by ester-bonding PLA (M-2)and pyromellitic acid, multi-functional carboxylic acid PLA (M-9) havinga number-average molecular weight of 100 to 100000 is obtained. By usingpyromellitic acid in large excess amount (10 to 100-fold mol) for ahydroxyl group, PLA in which pyromellitic acid is ester-bonded to oneend thereof is obtained. By refluxing PLA (M-2) and pyromellitic acid ina mixed solvent of chloroform and THF, or a toluene solvent, theesterification reaction can be progressed. Pyromellitic acid usedexcessively can be removed by washing with hot water after removal ofthe solvent.

(M-10) Multi-functional carboxylic acid PLA: by ester-bondingend-hydroxy PLA (M-3) and pyromellitic acid, multi-functional carboxylicacid PLA (M-10) having a number-average molecular weight of 100 to1000000 is obtained. By refluxing end-hydroxy PLA (M-3) and large excessamount (10 to 100-fold mod) of pyromellitic acid in a mixed solvent ofchloroform and THF, or a toluene solvent, the esterification reactioncan be progressed. Pyromellitic acid used excessively can be removed bywashing with hot water after removal of the solvent.

EXAMPLE 1-4

Diels-Alder Type Cross-Linked Resin

Cyclopentadienyl sodium and excess dry ice are reacted to obtaindicyclopentadienedicarboxylic acid. To this are added a carboxylic acidand oxaallyl chloride in equimolar or more amount in THF, to obtaindicyclopentadienecarboxylic chloride. The solvent is distilled off underreduced pressure at 60° C. Using this as a cross-linking agent, thepolyester resins (M-1) to (M-3) are reacted. Hydrochloric acid isremoved from a hydroxyl group of a polyester resin anddicyclopentadienecarboxylic chloride, and dicyclopentadienecarboxylicacid is ester-bonded to a hydroxyl group of a polyester resin.Resultantly, a dicyclopentadiene-cross-linked polyester resin isobtained using dicyclopentadiene as a cross-linking portion. Thereaction of removing hydrochloric acid progresses at normal temperatureunder a nitrogen atmosphere in a chloroform solvent, and re-precipitatedin a poor solvent, thus, a dicyclopentadiene-cross-linked polyesterresin can be recovered. Though the cleaving temperature of across-linked portion by dicyclopentadiene is 100 to 250° C.,temperatures for obtaining moldable flowability can be controlled by themolecular weight of a polyester resin used, the density of a hydroxylgroup and cross-linked density (amount of cross-linking agent used), andthe like.

EXAMPLE 1-5

Nitroso Dimer Type Cross-Linked Resin

Using a dimer of 4-nitroso-3,5-dichlorobenzoyl chloride as across-linking agent, the polyester resins (M-4), (M-5) and (M-6) havinga phenolic hydroxyl group are melt-mixed at 150 to 250° C., to obtainnitroso dimer type cross-linked resins having a nitroso dimer structureas a cross-linked portion. Though the cleaving temperature of across-linked portion by a nitroso dimer is 110 to 150° C., temperaturesfor obtaining moldable flowability can be controlled by the molecularweight of a polyester resin used, the density of a phenolic hydroxylgroup and cross-linked density (amount of cross-linking agent used), andthe like.

EXAMPLE 1-6

Acid Anhydride Ester Type Cross-Linked Resin

A di- or more-functional acid anhydride is used as a cross-linkingagent. As such an acid anhydride, for example, a copolymer (VEMAmanufactured by Daicel Chemical Industries, Ltd.) of maleic anhydrideand methyl vinyl ether having a weight-average molecule weight of900000, pyromellitic anhydride, 1,2,3,4-butanetetracarboxylic anhydride(manufactured by New Japan Chemical Co., Ltd., trade name: RikacidBT-100),(5-dioxotetrahydro-3-solanyl)-3-methyl-3-cyclohexene-1,2-dicarboxylicanhydride (manufactured by DIC, trade name: EPICLON B4400), and the likeare used. These acid anhydrides and the polyester resins (M-1) to (M-3)are reacted, and an ester bond is formed from a hydroxyl group of thepolyester resin, and the acid anhydride. Resultantly, an acid anhydrideester-cross-linked polyester resin is obtained using an ester bondobtained from the acid anhydride as a cross-linked portion. Theesterification reaction progresses by reflux under a nitrogen atmospherein a mixed solvent of chloroform and THF, or a toluene solvent, andre-precipitated in hexane, poor solvent, thus, an acid anhydrideester-cross-linked polyester resin can be recovered. Though the cleavingtemperature of a cross-linked portion by an acid anhydride ester is 100to 250° C., temperatures for obtaining moldable flowability can becontrolled by the molecular weight of a polyester resin used, thedensity of a hydroxyl group and cross-linked density (amount ofcross-linking agent used), and the like.

EXAMPLE 1-7

Halogen-Amine Type Cross-Linked Resin

Hydroxyl groups of the polyester resins (M-1) to (M-3) are ester-bondedto a carboxyl group of 4-bromomethylbenzoic acid, to obtain halogenatedpolyester resins. By reacting tetramethylhexanediamine as across-linking agent with these resins, halogen-amine type cross-linkedpolyester reins having an ammonium bond as a cross-linked portion isobtained. Though the cleaving temperature of a cross-linked portion by ahalogen-amine bond is 100 to 200° C., temperatures for obtainingmoldable flowability can be controlled by the molecular weight of apolyester resin used, the density of a hydroxyl group and cross-linkeddensity (amount of cross-linking agent used), and the like.

EXAMPLE 1-8

Urethane Type Cross-Linked Resin

Using toluene diisocyanate and phenylmethane diisocyanate and the likeas a cross-linking agent, the polyester resins (M-4), (M-5) and (M-6)having a phenolic hydroxyl group are melt-mixed at 150 to 250° C., toobtain urethane type cross-linked polyester resins having a urethanebond as a cross-linked portion. Though the cleaving temperature of across-linked portion by a urethane bond is 120 to 250° C., temperaturesfor obtaining moldable flowability can be controlled by the molecularweight of a polyester resin used, the density of a phenolic hydroxylgroup and cross-linked density (amount of cross-linking agent used), andthe like. Also by using a cleaving catalyst such as1,3-diacetoxytetrabutyl distannoxane and the like in an amount of 0.01to 1.0 part by weight per 100 parts by weight of the urethane typecross-linked resin, the cleaving temperature can be controlled.

EXAMPLE 1-9

Azlactone-Phenol Type Cross-Linked Resin

Using bisazlactylbutane and the like as a cross-linking agent, thepolyester resins (M-4), (M-5) and (M-6) having a phenolic hydroxyl groupare melt-mixed at 150 to 250° C., to obtain polyester resins having anazlactone-phenol bond as a cross-linked portion. Though the cleavingtemperature of a cross-linked portion by an azlactone-phenol bond is 100to 200° C., temperatures for obtaining moldable flowability can becontrolled by the molecular weight of a polyester resin used, thedensity of a phenolic hydroxyl group and cross-linked density (amount ofcross-linking agent used), and the like.

EXAMPLE 1-10

Carboxyl-Vinyl Ether Type Cross-Linked Resin

Using bis[4-(vinyloxy)butyl] adipate and the like as a cross-linkingagent, the polyester resins (M-7) to (M-10) having a carboxyl group aremelt-mixed at 150 to 250° C., to obtain carboxyl-vinyl ether typecross-linked polyester resins having a hemiacetal ester bond as across-linked portion. Though the cleaving temperature of a cross-linkedportion by a hemiacetal ester bond is 100 to 250° C., temperatures forobtaining moldable flowability can be controlled by the molecular weightof a polyester resin used, the density of a carboxyl group, addition ofan acid catalyst and cross-linked density (amount of cross-linking agentused), and the like.

EXAMPLE 1-11

Use of Electrostatically Bond for Cross-Linked Structure in Combination

The polyester resins (M-7) to (M-10) obtained above are melted at 100 to200° C., and ions are added. As the ion source (cation), Cu, Na, Mg, Caand the like are used. An aqueous solution of copper acetate, sodiumacetate, calcium acetate, magnesium acetate and the like is added sothat the neutralization degree is preferably 1% or more, more preferably10% or more and 100% or less, more preferably 95% or more, andimmediately, water is distilled off under reduced pressure. Though thecleaving temperature of a cross-linked portion is 100 to 200° C.,temperatures for obtaining moldable flowability can be controlled by themolecular weight of a polyester resin used, the density of a carboxylgroup, the neutralization degree of a carboxyl group by a metal ion, andthe like.

Thus obtained composition is mixed, for example, with theabove-mentioned carboxyl-vinyl ether type cross-linked resin, to use acovalently bondable cross-linked structure and a cross-linked structurewith electrostatically bonding in combination.

EXAMPLE 1-12

Use of Electrostatically Bondable Cross-Linked Structure in Combination

The above-mentioned carboxyl-vinyl ether type cross-linked resin ismelted at 100 to 200° C., and ions are added to this, to use acovalently bondable cross-linked structure and a cross-linked structurewith electrostatically bonding in combination. As the ion source(cation), Cu, Na, Mg, Ca and the like are used.

EXAMPLE 2-1

Diels-Alder Type Cross-Linked Biodegradable Resin 1

Using stannous octanoate as a catalyst in an amount of 0.05 parts byweight per 100 parts by weight of lactide (dimer of lactic acid), thelactide was ring-opening-polymerized at a reaction temperature of 200°C., to obtain PLA having a number-average molecular weight of 100000(C-1). To PLA (1000 g) was added glycerin (0.5 mol, 46 g), and atransesterification reaction was conducted at 180° C. for 6 hours. Thiswas dissolved in chloroform and washed with an alkali aqueous solution,then, the solvent was distilled off to obtain end-hydroxy PLA (C-2)having a number-average molecular weight of 4000.

Cyclopentadienyl sodium (1.6 mol THF solution, 1 L) and dry ice (2 kg)were reacted, to obtain dicyclopentadienedicarboxylic acid. To this wasadded oxaallyl chloride in equimolar or more amount in THF, to obtaindicyclopentadienecarboxylic chloride. The solvent was distilled offunder reduced pressure at 60° C. The above-mentioned end-hydroxy PLA(100 g) was dissolved in chloroform (3 L), anddicyclopentadienedicarboxylic chloride (0.038 mol) and equimolarpyridine were added and reacted at normal temperature for 24 hours,then, unreacted materials and impurities were removed by washing. Bydistilling off the solvent, a dicyclopentadiene-cross-linked polyesterresin was recovered (the number of cross-linked points capable offorming three-dimensional cross-linking is about 0.023 per 100 g of aresin substance).

EXAMPLE 2-2

Diels-Alder Type Cross-Linked Biodegradable Resin 2

Glycerin (2 mol, 184 g) was added to 1 mol of PLA (C-1) obtained in thesame manner as in the case of the Diels-Alder type cross-linkedbiodegradable resin 1, and a transesterification reaction thereof wasconducted at 180° C. for 6 hours. This was dissolved in chloroform, andwashed with an alkali aqueous solution, then, the solvent was distilledoff, to obtain end-hydroxy PLA (C-3) having a number-average molecularweight of 1000.

The above-mentioned end-hydroxy PLA (100 g) was dissolved in chloroform(3 L), and dicyclopentadienedicarboxylic chloride (0.15 mol) obtained inthe case of the Diels-Alder type cross-linked biodegradable resin 1 andequimolar pyridine were added and reacted at normal temperature for 24hours, then, unreacted materials and impurities were removed by washing.By distilling off the solvent, a dicyclopentadiene-cross-linkedpolyester resin was recovered (the number of cross-linked points capableof forming three-dimensional cross-linking is about 0.078 per 100 g of aresin substance).

EXAMPLE 2-3

Diels-Alder Type Cross-Linked Biodegradable Resin 3

According to the same manner as in the means of Chan-Ming D. et al.(Polymer, vol. 42, p. 6891, 2001), using trimethylolpropane in an amountof 0.16 parts by weight and stannous octanoate as a catalyst in theamount of 0.06 parts by weight per 100 parts by weight of lactide, thelactide was ring-opening-polymerized at 110° C. for 100 hours, to obtainPLA having a number-average molecular weight of 40000 (C-4).

The above-mentioned end-hydroxy PLA (100 g) was dissolved in chloroform(3 L), and dicyclopentadienedicarboxylic chloride (0.0038 mol) obtainedin the case of the Diels-Alder type cross-linked biodegradable resin 1and equimolar pyridine were added and reacted at normal temperature for24 hours, then, unreacted materials and impurities were removed bywashing. By distilling off the solvent, a dicyclopentadiene-cross-linkedpolyester resin was recovered (the number of cross-linked points capableof forming three-dimensional cross-linking is about 0.0025 per 100 g ofa resin substance).

EXAMPLE 2-4

Carboxyl-Alkenyloxy Type Cross-Linked Biodegradable Resin 1

100 g of PLA (C-2) obtained in the case of the Diels-Alder typecross-linked biodegradable resin 1 was dissolved in chloroform (3 L),and succinic anhydride (0.075 mol) and pyridine (0.05 g) as a catalystwere added, and they were refluxed for 6 hours. After the reaction,pyridine was extracted and washed, then, the solvent was removed, toobtain end-carboxylic acid PLA resin (C-4). Into 100 g of this resin(A-4) was melt-kneaded 11.7 g of tris[4-(vinyloxy)butyl] trimellitate asa linker by Mini Max Mix Truder (trade name) manufactured by Toyo SeikiK.K., to obtain a composition (the number of cross-linked points capableof forming three-dimensional cross-linking is about 0.034 per 100 g of aresin substance).

EXAMPLE 2-5

Carboxyl-Alkenyloxy Type Cross-Linked Biodegradable Resin 2

100 g of PLA (C-3) obtained in the case of the Diels-Alder typecross-linked biodegradable resin 2 was dissolved in chloroform (3 L),and succinic anhydride (0.30 mol) and pyridine (0.05 g) as a catalystwere added, and they were refluxed for 6 hours. After the reaction,pyridine was extracted and washed, then, the solvent was removed, toobtain end-carboxylic acid PLA resin (C-4). Into 100 g of this resin(C-5) was melt-kneaded 38.8 g of tris[4-(vinyloxy)butyl] trimellitate asa linker by Mini Max Mix Truder (trade name) manufactured by Toyo SeikiK.K., to obtain a composition (the number of cross-linked points capableof forming three-dimensional cross-linking is about 0.110 per 100 g of aresin substance).

EXAMPLE 2-6

Diels-Alder Type Cross-Linked Biodegradable Resin (not Three-DimensionalCross-Linking)

To PLA (C-1) obtained in the same manner as in the case of theDiels-Alder type cross-linked biodegradable resin 1 was added butanediol(0.5 mol, 45 g), and a transesterification reaction was conducted at180° C. for 6 hours. This was dissolved in chloroform, and washed withan alkali aqueous solution, then, the solvent was distilled off, toobtain end-hydroxy PLA (C-6) having a number-average molecular weight of3000.

The above-mentioned end-hydroxy PLA (100 g) was dissolved in chloroform(3 L), and dicyclopentadienedicarboxylic chloride (0.033 mol) obtainedin Example 1 was added, and they were reacted at normal temperature for24 hours, then, the solvent was distilled off to recover adicyclopentadiene-cross-linked polyester resin (containing nocross-linked points capable of forming three-dimensional cross-linking).

The above-mentioned evaluation results are shown in Table 2.

TABLE 2 Evaluation result three-dimensional cleaving heat recyclingmolding wet biodegradable resin cross-linked density temperatureresistance biodegradability propertiy property resistance Diels-Aldertype cross-linked 0.023 190° C. ⊚ ◯ ◯ ◯ 95% biodegradable resin 1Diels-Alder type cross-linked 0.078 190° C. ⊚ ◯ ◯ ◯ 95% biodegradableresin 2 Diels-Alder type cross-linked 0.0025 190° C. ⊚ ◯ ◯ ◯ 95%biodegradable resin 3 carboxyl-alkenyloxy type cross- 0.034 195° C. ⊚ ◯◯ ◯ 95% linked biodegradable resin 1 carboxyl-alkenyloxy type cross-0.110 195° C. ⊚ ◯ ◯ ◯ 95% linked biodegradable resin 2 Diels-Alder typecross-linking 0 190° C. ◯ ◯ ◯ ◯ 95% (no three-dimensional cross-linking)acid anhydride type cross-linked 0.060 180° C. ◯ ◯ ◯ ◯ 50% biodegradableresin (composition 3) A-1 0 none X ◯ ◯ ◯ 90% C-1 0 none X ◯ ◯ ◯ 99%

As apparent from Table 2, all biodegradable resins have sufficientabilities, and particularly, in the case of a resin containingintroduced three-dimensional cross-linking, heat resistance isparticularly high. When the cross-linked structure is a Diels-Alder typeor carboxyl-alkenyloxy type, heat resistance and moisture resistance arehigh.

1. A biodegradable moldable resin having a Diels-Alder type functionalgroup selected from the group consisting of an alkenyl group and a grouphaving a conjugated double bond wherein said biodegradable moldableresin is polybutylene succinate or modified body of the polybutylenesuccinate.
 2. The biodegradable moldable resin according to claim 1,wherein said biodegradable moldable resin has a three-dimensionalcross-linked structure, and the cross-linked density of thethree-dimensional cross-linked structure is 0.0001 to
 1. 3. Thebiodegradable moldable resin according to claim 1, wherein the mainchain of said biodegradable moldable resin has at least one of a linearstructure and branched structure.
 4. The biodegradable moldable resinaccording to claim 1, wherein one or more of said functional groups arepresent at the same site, at least one of the end and side chain of saidbiodegradable moldable resin.
 5. The biodegradable moldable resinaccording to claim 1, wherein an electrostatically bondable andthermo-reversible cross-linked structure is used together.
 6. Thebiodegradable moldable resin of claim 1 wherein the functional group isselected from the group consisting of cyclic dienes and cyclicdienophiles.